Aural stereophonic balance indicator



Dec. 7, 1965 E. R. GOLIK 3,222,455

AURAL STEREOPHONIC BALANCE INDICATOR Filed May 51. 1962 INVENTORI EDWINR.GOL|K QWAM m HIS ATTORNEY.

United States Patent M 3,222,455 AURAL STEREOPHONIC BALANCE INDICATOREdwin R. Golik, Oreana, Ill., assignor to General Electric Company, acorporation of New York Filed May 31, 1962, Ser. No. 198,834 4 Claims.(Cl. 179-1) This invention relates to means for balancing the gain oftwo audio channels.

In a stereophonic-audio system, two separate amplifying channels, eachenergizing an acoustic transducer, are provided. In order to insureoptimum performance in such a system it is necessary that the gains ofthe channels be equalized. The achievement of this goal has beeneffected in most prior art systems by expensive balance indicators.

It is an object of this invention to provide means whereby the gains oftwo channels can be equalized by the addition of a minimum number ofcomponents.

Briefly, this objective is achieved by the provision of a switch thatconnects the outputs of the two channels in a series circuit in suchrelative polarities that the voltages in the outputs tend to cancel.When the channel gains are adjusted so as to be equal, the outputvoltages of the channels cancel, and an aural null is produced in thesounding emanating from the acoustic transducers for each channel.

It has been discovered that with some amplifier designs, the balancingcircuit set up by the switch may cause the amplifier to oscillate as thedesired aural null is approached, thus making it difiicult to secure thedesired balanced condition.

Accordingly it is another object of this invention to provide abalancing circuit of the type described wherein the tendency foroscillation to occur is greatly reduced.

Reference is now made to the drawing for a detailed explanation of oneform of this invention. Input terminals 2 and 4 are respectivelyconnected to ungrounded ends of balancing potentiometers 6 and 8. Themovable contact arms 10 and 12 are ganged together in such a manner thatone moves farther from ground as the other moves toward it and viceversa, as indicated by the arrows. Amplifiers 14 and 16 have theirinputs respectively connected to primary windings 18 and 20 of outputtransformers 22 and 24. Positive direct current operating potential issupplied to the amplifiers 14 and 16 by connection of a suitable sourcebetween ground and the ends of the secondary windings 18 and 20 that areremote from the amplifiers. Secondary windings 26 and 28 of thetransformers 22 and 24 have ends of like polarity connected to a pointof reference potential such as ground. When a switch 30 is closed, loadimpedances, herein shown as voice coils 36 and 38 for loudspeakers 40and 42, are connected in parallel with the secondary Windings 18 and 20.

When the switch 30 is closed, the electrical connections areconventional, but the provision of a switch which enables the groundconnection to the junction of the voice coils 36 and 38 to be broken isnovel and is a part of this invention and operates in the followingmanner. To balance the relative gains of the two channels, the switch 30is opened, as shown, and if the signals applied to the input terminals 2and 4 are not known to be identical a switch 44 is closed so as toconnect the terminals 2 and 4 together and thus apply identical signalsto the potentiometers 6 and 8. For ease in operation, the switches 44and 30 can be mechanically connected so that closing one 3,222,455Patented Dec. 7, 1965 opens the other. With the switch 30 in an openpostion, the secondary windings 26 and 28 and the voice coils 36 and 38form a closed loop in which the audio voltages inducted in the secondarywindings 18 and 20 oppose each other. When the gains of the channels areunequal, one of these induced audio voltages will be greater than theother with the result that sound will emanate from both of theloudspeakers 40 and 42. The volume of the sound increases with thedegree of imbalance. Equating the gains of the two channels with a meanssuch as the balance control potentiometers 6 and 8, causes the audiovoltages induced in the secondary windings 26 and 28 to be substantiallyidentical. In this conditon substantially no current flows in the loop26, 28, 36 and 38, and hence the loudspeakers receive no energy. Thus,when the gains are balanced, an aural null is produced in the soundemanating from the loudspeakers 40 and 42.

Upon identical balance being achieved, and with switch 30 in the openposition, the load impedance looking into the output secondary windings26 and 28 from their respective primary windings appears to be infinite,thus creating the possibility that the amplifiers 14 and 16 will becomeunstable. Insertion of load resistors 32 and 34 sustains a simulatedload impedance for the secondary windings 26 and 28 and reduces thetendency of the amplifiers 14 and 16 to become unstable and oscillate.

It will be understood by those skilled in the art that the polarities ofthe secondary windings 26 and 28 indicate the polarities of the audiosignal voltages produced across the secondary windings where the samesignal is applied to each of the terminals 2 and 4. If the channels areidentical, the polarities are the same as the windings sense of thesecondary windings 26 and 28 with respect to the primary windings 18 and20.

What I claim is:

1. A circuit for aiding in indicating where the gains of two separateaudio signal channels are in balance comprising a first channel having afirst output transformer, a second channel having a second outputtransformer, each of said transformers having a secondary winding with apredetermined polarity, means electrically connecting an end of one ofsaid secondary windings to an end of like polarity of the other of saidsecondary windings, first and Second load irnpedances, and a single polesingle throw switch connected so that said first and second loadirnpedances are respectively connected in parallel with said secondarywindings when said switch is closed and said secondary windings and saidload impedances are connected in series when said switch is open.

2. A circuit as set forth in claim 1 wherein load resistors areconnected in shunt with each of said secondary windlngs.

3. The combination of a first signal amplifying channel having a pair ofoutput terminals, a second signal amplifying channel having a pair ofoutput terminals, first and second load impedances, switching means foralternatively connecting said first and second load impedancesrespectively in parallel with said output terminals or connecting saidoutput terminals and said first and second load irnpedances in a seriesloop, and a resistor connected across each of said pairs of outputterminals.

4. The combination of a first amplifier having an input and an output, asecond amplifier having an input and an output, a first outputtransformer having a primary and a secondary winding connections placingsaid primary winding of said first output transformer across said outputof said first amplifier, a sec-0nd output transformer having a primaryand a secondary winding, connections for placing said primary winding ofsaid second output transformer across said output of said secondamplifier, a point of reference potential, connection between said pointand one end of each of said secondary windings having signals of likepolarity, a first loudspeaker having a first voice coil, a secondloudspeaker having a second voice coil, a connection between one end ofsaid first voice coil and the other end of said secondary winding ofsaid first output transformer, a connection between one end of saidsecond voice coil and the other end of said secondary winding of saidsecond output transformer, a con nection between the other ends of saidfirst and second voice coils, and a single pole single throw switchconnected between said point of reference potential and said other endsof said first and second voice coils.

4 References Cited by the Examiner UNITED STATES PATENTS 769,638 9/1904Sargent 31591 2,393,885 1/1946 Claassen 1791 2,882,452 4/1959 Bembenek31591 3,001,019 9/1961 Uecke et a1. 179-1 FOREIGN PATENTS 1,291,584-3/1962 France.

OTHER REFERENCES Burstein et al.: Stereo Balance and Gain Control; Radio& TV News; December 1957, pp. 42, 43, 106.

ROBERT H. ROSE, Primary Examiner.

WILLIAM C. COOPER, Examiner.

1. A CIRCUIT FOR AIDING IN INDICATING WHERE THE GAINS OF TWO SEPARATEAUDIO SIGNAL CHANNELS ARE IN BALANCE COMPRISING A FIRST CHANNEL HAVING AFIRST OUTPUT TRANSFORMER, A SECOND CHANNEL HAVING A SECOND OUTPUTTRANSFORMER, EACH OF SAID TRANSFORMERS HAVING A SECONDARY WINDING WITH APREDETERMINED POLARITY, MEANS ELECTRICALLY CONNECTING AN END OF ONE OFSAID SECONDARY WINDINGS TO AN END OF LIKE POLARITY OF THE OTHER OF SAIDSECONDARY WINDINGS, FIRST AND SECOND LOAD IMPEDANCES, AND A SINGLE POLESINGLE THROW SWITCH CONNECTED SO THAT SAID FIRST AND SECOND LOADIMPEDANCE ARE RESPECTIVELY CONNECTED IN PARALLEL WITH SAID SECONDARYWINDINGS WHEN SAID SWITCH IS CLOSED AND SAID SECONDARY WINDINGS AND SAIDLOAD IMPEDANCES ARE CONNECTED IN SERIES WHEN SAID SWITCH IS OPEN.